With a recent increasing demand for lightweight or higher output of automobiles for the purpose of a decrease in exhaust gas or improvement of fuel efficiency, high stress design has also been required for valve springs, clutch springs, suspension springs and the like which are used in engines, clutches, suspensions and the like. These springs tend to have higher strength and thinner diameter, and the load stress tends to further increase. In order to comply with such a tendency, a spring steel having higher performance in fatigue resistance and settling resistance has been strongly desired.
Further, in order to realize lightweight while maintaining fatigue resistance and settling resistance, hollow pipe-shaped steel materials having no welded part (that is to say, seamless pipes) have come to be used as materials of springs, instead of rod-shaped wire rods which have hitherto been used as materials of springs (that is to say, solid wire rods).
Techniques for producing the hollow seamless pipes as described above have also hitherto been variously proposed. For example, Patent Document 1 proposes a technique of performing piercing by using a Mannesmann piercer which should be said to be a representative of piercing rolling mills (Mannesmann piercing), then, performing mandrel mill rolling (draw rolling) under cold conditions, further, performing reheating under conditions of 820 to 940° C. and 10 to 30 minutes, and thereafter, performing finish rolling.
On the other hand, Patent Document 2 proposes a technique of performing hydrostatic extrusion under hot conditions to form a hollow seamless pipe, and thereafter, performing spheroidizing annealing, followed by performing extension (draw benching) by Pilger mill rolling, drawing or the like under cold conditions. Further, in this technique, it is also shown that annealing is finally performed at a predetermined temperature.
In the respective techniques as described above, when the Mannesmann piercing or the hot hydrostatic extrusion is performed, it is necessary to heat at 1,050° C. or more or to perform annealing before or after cold working, and there is a problem that decarburization is liable to occur in an inner peripheral surface and outer peripheral surface of the hollow seamless pipe at the time of heating or working under hot conditions or in a subsequent heat treatment process. Further, at the time of cooling after the heat treatment, decarburization (ferrite decarburization) caused by the difference between the solute amount of carbon in ferrite and that in austenite also occurs in some cases.
When the decarburization as described above occurs, it happens that surface layer parts are not sufficiently hardened in the outer peripheral surface and inner peripheral surface in a quenching step at the time of spring production, which causes a problem that sufficient fatigue strength cannot be secured in springs to be formed. Further, in the case of usual springs, residual stress is usually imparted to an outer surface by shot peening or the like to improve the fatigue strength. However, in the case of springs formed by the hollow seamless pipe, shot peening cannot be performed in the inner peripheral surface, and flaws are liable to occur in the inner peripheral surface by a conventional processing method. Accordingly, there is also a problem that it becomes difficult to secure the fatigue strength of the inner surface.